Botfly Infection in Horses

Bot Parasite Infestation in Horses

Botflies are an unfortunate byproduct of caring for horses. They are a frequent source of irritation for horses, especially during the hot months of late summer, when these flies seem to be always around.

The larvae of the botfly is referred to as a bot, and a horse that is infested with botfly larvae is said to have bots. The botfly begins by laying eggs on the outer body of the horse – on the skin of the inner legs and knees, around the chin and nose, and on the belly. In appearance, the adult botfly resembles a scrawny honeybee, with light hair on the thorax and yellowish coloring. The eggs are small and yellow-orange in color, and are attached to the hairs of the horse's body by the adult botfly. The horse then licks or bites the spot where the eggs are.

In this way the larvae are transported to the horse's mouth, where they remain for about four weeks before migrating to the digestive system. The larvae stay in the digestive tract to mature until they are ready to leave the body through the fecal contents. They then burrow into the ground to mature into adults. The entire process takes place from one season to the next, with one generational cycle taking place each year. The emergence of the adult botfly from the ground marks the beginning of the next cycle. In most states, the botfly is a seasonal nuisance that takes place from Spring through late fall, but in South Florida, the botfly has been found to remain active throughout the year.

Botfly infestation has some zoonotic characteristics, meaning that it can infect humans as well as animals. Particular care must be used when cleaning an infected horse, during removal of eggs, and afterwards during clean-up.

Symptoms and Types

• Three types of botflies:
  • Common horse bot (Gastrophilus intestinalis): eggs are laid on body, taken into mouth while self grooming
  • Throat bot (Gastrophilus nasalis): eggs are laid on neck and beneath jaw, larvae make their way into horse's mouth
  • Nose bot (Gastrophilus haemorrhoidalis) – rare: eggs are laid around lips
• Clumps of eggs on horse's body - may be orange or yellow in color
• Presence of large flies hovering near eyes, ears, and rump of the horse
• Licking of the stomach and legs
• Rubbing face or biting objects to relieve irritation in mouth
• Ulcers in and around mouth, pus pockets in gums, loose teeth
• Paralysis of the esophagus – unable to drink or eat in quantity
• Colic: large numbers of larvae in the intestine or stomach may cause blockage or stomach ulcers – symptoms include diarrhea, soft stools, poor appetite
• Abdominal discomfort or pain from gastritis, peritonitis, or stomach rupture
• Anemia: lethargy, weight loss

Causes

• Eggs are laid on exterior of horse in the spring - fall months and left to migrate and incubate in horse intestines
• Self-grooming activities encourage eggs to hatch - larvae are ingested when horse licks and grooms itself
• Fly larvae incubate in stomach lining for up to ten months before migrating out of the body
• Larvae is passed through feces into the pasture or stable grounds, where horse is reinfected, or other horses are infected

Diagnosis

A veterinarian can diagnose bots in a horse by a microscopic examination of its feces. The larvae may also be found in the soil at pasture in places where there are multiple horses. The color of the eggs also makes them rather easy to spot as they are yellow and orange in color, a bright contrast against the skin color of most breeds of horse. You veterinarian may also find evidence of bot infestation by examining your horse's teeth and gums for ulcers, pus pockets, or loose teeth.